1. Field of the Invention
This invention relates to a silicone fluid-based anti-offsetting oil used in fixing rolls.
2. Description of the Prior Art
In electrophotographic copying, as a method for heat fixing toner images formed of thermoplastic powder which have been transferred to copying sheet, it is common to bring a heated roll into direct contact with toner images. However, once the toner images turn viscous upon heat melt, a part of toner images on copying sheet such as copying paper may adhere also to the surface of the roll, so that this part of toner images is transferred from a first sheet to a next sheet when the next sheet is delivered onto the roll, and simultaneously a part of viscous toner images transferred from the next sheet adheres to the roll. This phenomenon is commonly called "offset" in printing techniques.
To solve this problem, a fixing roll has been conventionally used which comprises a core roll covered on its surface with a film having a low surface energy as exemplified by polytetrafluoroethylene (PTFE) film or tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer (PFA) film on which silicone fluid is applied in a thin film at the time of heat fixing (Japanese Post-examination Patent Publication (Kokoku) No. 41-16990). In such a fixing roll, however, the roll surface has so low a surface energy that the silicone fluid applied may be repelled and no uniform coating can be formed, resulting in no sufficient anti-offset effect. As a method for improving such a fixing roll, it is proposed to use a silicone fluid to which a fluorine-containing surfactant is added (Japanese Pre-examination Patent Publication (kokai) No. 58-11624). However, because of a poor compatibility of silicone fluid with the fluorine-containing surfactant, no stable anti-offset effect has been achieved.
As an anti-offsetting silicone fluid, a silicone fluid having a fluorine-containing organic group in the molecule is also proposed (Japanese Pre-examination Patent Publication No. 51-18544). However, no sufficient effect can be achieved unless an organopolysiloxane having a long-chain fluorine-containing organic group such as a fluoroalkyl group or a fluoropolyether group is used. However, for reasons of production, such an organopolysiloxane having a long-chain fluorine-containing organic group can not avoid containing low-molecular weight volatile components, which are causative of electric-contact failure of electronic devices. For example, a linear organopolysiloxane production process comprising the step of subjecting a cyclotrisiloxane having a long-chain fluorine-containing organic group to polymerization in the presence of a catalyst such as concentrated sulfuric acid or activated clay is accompanied by depolymerization reaction, and this reaction produces as by-products volatile components such as cyclic siloxanes, in particular, cyclic oligosiloxanes with three to tens of siloxane units or a low-molecular weight linear siloxane. As known in the art, when the linear organopolysiloxane containing such volatile components is used as an anti-offsetting oil, the volatile components in this organopolysiloxane gradually volatilize to adhere to electric contacts and so forth to cause troubles such as contact failure. Accordingly, when the linear organopolysiloxane is used as an anti-offsetting oil, the content of the volatile components in it must be made as small as possible. Of the above volatile components, the low-molecular weight linear siloxanes and the cyclic siloxanes having a low degree of polymerization, i.e., having a low molecular weight can be removed by subjecting the resultant polymer to stripping under reduced pressure and heating or solvent extraction. It, however, is very difficult to remove cyclic siloxanes with a polymerization degree of 10 or more, in particular, with 20 or more. When the linear organopolysiloxane which has been thus treated is used as the anti-offsetting oil, the troubles such as contact failure can not be avoided because it contains as volatile components cyclic products with a polymerization degree of 10 or more and a molecular weight of not more than 3,000 in a rather large quantity.
For the above reasons, it has been sought to develop a silicone fluld-based anti-offsetting oil having a good wettability to low-surface energy materials such as silicone rubber and fluorine rubber and also containing no organopolysiloxane having a molecular weight of 3,000 or less including cyclic siloxanes with a polymerization degree of 20 or more.